Optical projection system



m nu ESU QB R. E. REASON OPTICAL PROJECTION SYSTEM ol -i inal Filed June 1, 1938 Alfome l aitented, July 25 1944- earanr caries or'ncar. rnomc'rIoN-srsrrm Richard Edmund Reason, Leicester, England, as-

Signor to Kapella Limited, Leicester, England, a company of Great Britain rig inal application runs. 1, 1938, Serial No. 2 11,231. {livid-ed and this application June 15, 1939, Serial No. 219,253. In GreatBr-itain Septcmber 24, 1937 2 Claims,

This application is divis'lonalirom application Serial No. 211,231 filed June 1-,j 19,38, which has become U. S.,Patent No. 2,256,102, dated Sept. ,16,

The invention relates to the optical projection 6 through a finite distance of "an image of an object having parts inclined to the direction of projection on to an image surface also having parts inclined to the direction of projection. With an ordinary projecting lens system; if used for projecting from an' object plane on to an image plane with both planes inclined to the optical axis of the projection system. the image will be distorted, more especially by what is known as The invention may be carrie into practice invarious ways, but some convenient practical artenements-according thereto are illustrated diagram'matically by way of example in the acoom'.

panying drawing, in which Figure 1 illustrates a preferred oblique projecting lens system for giving a rectified image,

Figures 2 and 3 show modifications thereof,

and

Figures and 5 show extensions of the system of Figure l to facilitate direct viewing.

In the preferred arrangement of Figure 1, the lens system comprises a projecting lens A and two collimating lenses B B spaced apart symkeystone" distortion. Thus, for example, equal metrically one on either side of the projectin distances from a line in the object plane perpenljdicular to the line of intersection of the image and object planes will in general be projected as unequal distances in the image plane, as also will the distortion, it is inconvenient in practice to lens. The projecting lens A itself is preferably H of the anastigmat or rectilinear type and may consist of a divergent component disposed between two convergent components symmetrical- Whilst lily ly arranged, with its nodal points A li in the air gaps between the components.

The two collimating lenses B B, each of which may consist of an achromatised triplet, are of equal focal lengthf'and the principal focus of it adopt such cxpedients. each lens coincides with the nodal point A or The present invention has for its object to pro- A of the projecting lens relating to the space in plication.

v'ide an optical projecting apparatus for oblique projection, by means of which a rectified image is obtainedfree from keystone distortion and which the collimating lens lies. Each collimak ing lens is spherically corrected with rape ct to such nodal" point (taking into account if necespreferably free from all distortion. Such an im- -sari the convergent component of the projecting age can be enlarged by normal projection in an enlarging projector.

The desired oblique projection is eifected ac-- cording to the invention by means of a system of lens betweenthe collimating lens and the nodal point), and fulfils the sine condition within th lenses so arranged as to colllmate the principal for the aberrations, particularly astigmatism rays'in the object and image spaces, that is to and Curvature o field- J Bf d by the 1 record the image photographically or to project it on to a diffusing screen, it will sometimes be desired to view the image directly through an mating lenses.

The whole system is symmetrica and produces an inverted image at unit magnification, theinn.

40 age and object planes CD being equally inclined to the optical axis. Slight inequality in the pow ers of the collimating' lenses may, however, be compensated for by a slightly asymmetrical arrangement, in which the central lens A is diseyepiece, and a further object of the invention is Pl s Weaker 0f the 0 so to extend the rectifying oblique projecting system as to facilitate such eyepiece viewing.

\ Although of general applicability toall cases where rectified'oblique projection is desired, the

lenses.

With this arrangement the principal rays from points in the object plane D to corresponding points in the image plane C are parallel to the invention is especially advantageous for use in optical axis in the spaces outside the collimating the optical measuring or testing apparatus of application Serial No. 211,231 above mentioned, and the manner in which it can be used therein is iully described inthe specification of such aplenses, and it will be clear that this condition ensures that a rectified image free from distortion due to the oblique. projection will be obtained. Such an image can cesatisfactorily enlarged bymeans of ordinary enlarging apparatus having its axis perpendicular to the image plane, pro.- vided that the surface of the plate or screen on which the image is projected is a true. fiat sur- 7 face.

It is not essential to the system to employ a projecting lens between the two collimating lenses, and such lens can be omitted altogether I if the object and imageplanes pass respectively I throughthe foci of the two collimating lenses.

'. The provision of the central lens, however, has

the advantage of bringing the two planes closer together and this in turn facilitates correction of theaberrations.

the collimating lenses, without disturbing either focussing or magnification, and the system will act to remove substantially all distortion due to oblique projection, it will be appreciated that in some instances it will suflice to remove'only keystone distortion and to depart from unit magnification. This may be done by using collimating lenses of different focal length or by displacing the central lens from the centre of the systern so that itadds to the power of one of the '30 collimating vlenses. provided that a focal adjustment is maintained. In this case the necessary angular relationship to ensure accurate locussing at the desired'magmncation' is that the ratio of the tangent of the angle between the object plane and'the optical axis to that of the angle between the image plane and the optical axis should be equal to the magnification of the sys-' tem. The image will, however, suffer from rectangular distortion, which can be compensated '40 for by the use of an anamorphotic enlarging system, including one or more cylindrical lenses. Even without such: compensation, a rectangularly distored image may itself be adequate, for

example in cases where the relative dimensions images Figures 2 and 3 show alternative arrangements wherein a rectifledimage free from keystone and rectangular distortion canbe btained at ma nifications other than unity. This isghieved by the use oi tilted lenses.

The simple arrangement .of Figure 2 comprises a projecting lens E and two collimating lenses F F of different focallength, one on either side V thereof. The collimating lenses are so spaced.

from the projecting lens thattheir principal focal points are approximately coincident with 0 the nodal points of the projecting lens." Each collimating lens is mounted squareto the opticaiaxis of the system, i. c. has its optical axis coincident with such axis, and the projecting lens is tilted at such an angle to the ends that the obc5 ject, which lies in a plane G inclined to the opti- I cal axis, is imaged'ina plane H equally inclined to the axis. The angle of tilt-of the projecting lensE necessary to produce this result can best be ex-,

plained by first considering the conditions which would arise if the projecting lens were not tilted (as indicated in dotted lines). In such a case i. e. the virtual image of the object formed by the collimating lens F and'the plane H containing the second virtual image (i. e. the image of the first virtual image which is formed by the projecting lens E and from which the final real image H is formed by the second collimating lens F intersect in the nodal plane of the projecting lens.

the position ofthe point of intersection of the plane ii of the second virtual image and the optical axis, and the extract of such tilting. will therefore be-to rotate the plane H 'of the second virtual image about such point 'of intersection into a position H The projecting lens should therefore be tilted to..such an angle as to maize the virtual image planes G: and H and therefore also the object and image planes GH,

equally inclined to the optical axis.-

In practice the arrangement of Figure 2 will require modification in order to provide correction for spherical and other aberrations. For this purpose the projecting lens E is preferably of the ,anastigmat or. rectilinear type and ma consist of a divergent component disposed between two convergent components symmetrically arranged with its nodal points in the air gaps between the components. In view of the tilting of the lens,

the two parts of the optical axis of the system with this arrangement the principal rays frompoints in the object plane to corresponding points in the image plane will be parallel to the optical axisin the spaces outside the collimating lenses, and it will be clear that this condition ensures that a rectified image free from distortion due to the oblique. projection will be obtained.

This arrangement may be modified by tilting the collimating lenses, either instead of or as well as the projecting lens. In such a. modification the two collimating lenses should be tilted through the same angle in order to avoid keystone distortion, and in' one convenient arrangement, as shown in Figure 3, all three lenses J. K, K, are equallytilted. In such cases the collimating lenses 3 K, if tilted through an appreciable angle, should preferably have the form of anastigmat lenses, special regard being paid to the correction of the aberrations within the field subtended by the projecting lens. The

focal lengths of the collimating lenses are measured normally to their nodal planes from the corresponding nodal points of the projecting lens, or in other words, their principal focal points lie in the corresponding nodal planes of the projecting lens. The nodal point of the projecting lens corresponding to a collimating lens need notbe the nodal point nearer to such lens, and in the example illustrated the nodal points-are crossed. The arrangement of Figure 3 has the adyantage that appreciable magnification can be obtained without excessive tilting l of the lenses, the object and image planes i again being equally inclined to the optical axis f of the system, which is displaced parallel to itself in its passage through each lens owing to Tilting of the projecting lens about its nodal point will not substantially alter the separation of the nodal points of the lens.

The ioregoing' arrangements are suitable for projection of the image on to a photographic plate or a projection screen or other surface,

but not for direct viewing through an eye-piece N, a plane mirror N being shown in the path of the rays to the eye-piece. If it is desired to 2,s cs4,e14- l 3 turbing to attempt to make measurements on an imagesuflering from keystone distortion, it ispreierable to provide a collimating field lens R between the surface C and the projecting lens 0. so that the final image will sufler only from rectangular distortion. In this case. as shown.

effect measurements of the images graticule may be provided in the image plane of the eyev piece so as to be superimposed on the image; This arrangement .is satisfactory providecl thatv the object can be sufficiently brightly illuminated to compensate for the loss of light d'ue to the diffusing screen;

This diihculty is obviated in the arrangement of Figure 5, in which a rectified image is obtained by the lens system of Figure 1' on a transparent surface C inclined to the optical axis, and a further projecting lens 0 is used to produce an lens 0. For simplicity in the drawing, the two nodal planes of the lens are assumed to be coincident. The final image will be sharply focussed, but will sufi'er from distortion, and in order to enable accurate measurements to be' made through the eye-piece, notwithstanding such parent surface 0, so that in the final image the object and the graduations will be equally distorted; Since it would in practice be rather dis;

formed by the field lens R ofthe image (7. The

the formation of the final image and can be corrected in association with the tilted projecting lens 0.

What I claim as my invention and desire to secure by Letters- Patent is:

1. Optical projection apparatus, comprising a system of lenses so arranged as to collimate the principal rays in the object and image spaces and having its optical axis so disposed that the ratio between the tangents of the angles made by such axis to the object and image planes is equal to the magnification of the system, a diii'using screen located in the image plane to receive the image, an eyepiece for viewing the image in a. direction at right angles to the diffusing screen, and a. graticule in the image plane of the eyepiece, whereby the image and the graticule will appear superimposed,

2. Optical projecting apparatus, comprising a symmetrical optical system for projecting I rectified image of an object lying in: a plane inclined to the optical axis of the system on to an image plane equally inclined to such axis, a difhzsing screen located in the image plane to receive the image, an eyepiece ior' viewing the image in a. direction at right angles to the diffusing screenaud a graticule in the image plane of distortion, graduations are provided on the trahsthe eyepiece, whereby the image and Braticu-le will appear superimposed, the symmetrical optical system consisting oi a projecting lens disposed between .two collimatin g lenses whose principal focal points respectively coincide with the nodal points of the projecting lens.

RICHARD EDMUND REASON. 

